1 Field of the Invention
This invention relates to a semiconductor laser drive circuit for supplying an electric current to a semiconductor laser, and more particularly to such a semiconductor laser drive circuit which is improved in the method of driving the semiconductor laser upon turning on the power source. This invention further relates to a light beam scanning system using a semiconductor laser.
2. Description of the Related Art
As is well known, a semiconductor laser has been in wide use as a read-out light source of a pickup for a compact disc player, a mini-disc player or the like or a stimulating light source for a radiation image read-out apparatus using a stimulable phosphor sheet or a fluorescence diagnosis apparatus or the like. As disclosed, for instance, in Japanese Unexamined Patent Publication No. 1(1989)-79695 and U.S. Pat. No. 5,530,261, the radiation image read-out apparatus using a stimulable phosphor sheet is a kind of medical image readout apparatus in which a stimulable phosphor sheet is used as a recording medium for recording a radiation image. That is, the stimulable phosphor sheet is provided with a layer of stimulable phosphor which, when exposed to a radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, stores a part of the energy of the radiation, and emits light in proportion to the stored energy of the radiation when exposed to stimulating light such as visible light, laser beam or the like. The stimulable phosphor sheet is exposed to radiation passing through an object and then exposed to a stimulating light to emit light in proportion to the stored radiation energy. The light emitted from the stimulable phosphor sheet is detected and converted to an electric image signal, on the basis of which a radiation image is reproduced. As disclosed, for instance, in Japanese Unexamined Patent Publication Nos. 1(1989)-136630 and 7(1995)-59783, the fluorescence diagnosis apparatus is a kind of medical image read-out apparatus in which a photosensitive material which has affinity to tumor and generates fluorescence upon stimulation by stimulating light having a certain wavelength is given to tumor and the tumor is exposed to stimulating light. Then an image formed by the fluorescence is used in diagnosis.
A semiconductor laser has been increasingly used as a light source for those systems in place of a vacuum tube type laser due to its compactness, inexpensiveness and easiness to handle.
However since the semiconductor laser is broken by an excess current, the electric current supplied to the semiconductor laser must be controlled not to exceed a maximum allowable current, for instance, by constant-current control. Even a momentary excess current, which can generated upon starting the semiconductor laser, especially upon turning on the power supply, should be prevented.
Conventionally the semiconductor laser is protected from a transient excess current upon turning on the power supply by providing the power supply circuit with a slow start circuit so that the supply voltage is gradually increased.
In the method of protecting the semiconductor laser from a transient excess current upon turning on the power supply by providing the power supply circuit with a slow start circuit, the starting time of the power supply circuit is controlled by use of charging properties of an integrating circuit formed by a resistor and an electrolytic capacitor. On the other hand, a conventional semiconductor laser drive circuit is provided with a control circuit such as an automatic current control circuit or an automatic power control circuit to prevent an excess current from being supplied to the semiconductor laser even in a steady state. As shown in FIG. 4, the automatic current control circuit uses an operational amplifier 24 as a control amplifier. A current control element 12 such as a transistor or a field effect transistor and a current detecting resistor 14 are connected in series with a semiconductor laser 10 so that an amount of current flowing through the semiconductor laser 10 is detected through the voltage across the resistor 14, and the detected voltage is input into an inverted input terminal (-) of the control amplifier 24 while a predetermined reference voltage is input into a non-inverted input terminal (+) of the control amplifier 24. The output of the control amplifier 24 is input into a control input terminal of the current control element 12. Thus a negative feedback circuit for controlling the current supplied to the semiconductor laser 10 to a predetermined value not larger than the maximum allowable current is formed. The automatic power control circuit is basically the same in structure as the automatic current control circuit as shown in FIG. 5 and differs from the automatic current control circuit in that the light output of the semiconductor laser 10 is detected by a photodetector 11 such as a photodiode and the current supplied to the semiconductor laser 10 is controlled so that the light output is controlled to a predetermined value not larger than the maximum allowable level.
Conventionally in such control circuits, the positive and negative power supplies of the operational amplifier are turned on substantially simultaneously. Accordingly even if the starting time of the power supply for the semiconductor laser is controlled by the slow start circuit, an excess current can transiently flow into the semiconductor laser depending on the starting state of the operational amplifier. This is because the negative feedback circuit including the operational amplifier behaves upon starting the power supply for the operational amplifier in a manner different from that of the steady state. This problem can be overcome by inserting an integrating circuit formed by a resistor and an electrolytic capacitor between the output of the operational amplifier and the control input terminal of the current control element so that current start of the current control element is delayed.
However even if such an integrating circuit is used, it is difficult to prevent an excess current from flowing into the semiconductor laser during a transient period from initiation of starting to the time the steady state is achieved. Further when the power supply is repeatedly turned on and off, it is very difficult to control the charging/discharging properties of the integrating circuit formed by a resistor and an electrolytic capacitor to prevent an excess current from flowing into the semiconductor laser.